This grant will investigate the central hypothesis that non-toxic doses of sphingolipids down-regulate cytosolic and nuclear beta-catenin, thereby lowering the expression of growth-promoting proteins and subsequently normalize cell growth and prevent tumor formation. This hypothesis has been formulated based on our preliminary in vivo data showing a suppression of tumor formation by dietary sphingolipids in mice, a modulation of beta-catenin localization and a reduction of the proliferation rate to a normal level. The objective of this proposal is to determine the importance of beta-catenin regulation by low doses of sphingolipids in the normalization of cell growth and, ultimately, in the prevention of colon cancer. The central hypothesis will be tested by pursuing three specific aims: 1) Identify the pathways of how low doses of exogenous sphingolipids down-regulate cytosolic beta-catenin. The working hypothesis is that exogenous sphingolipids lower cytosolic beta-catenin by enhancing degradation (early event) and suppressing transcription (late event). Key events in the proteasomal beta-catenin degradation and transcription will be evaluated. 2) Determine the mechanism of beta-catenin regulation by exogenous sphingolipids. The working hypothesis is that sphingolipids regulate beta-catenin metabolism indirectly via serine/threonine kinases and phosphatases that alter beta-catenin phosphorylation and thereby its fate. Changes in the activity of key protein kinases (i.e, PKC isozymes) and phosphatases (i.e., PP2A) will be quantitated and correlated to beta-catenin expression and localization. 3) Identify down-stream targets of low doses of sphingolipids and assess the relevance for sphingolipid-mediated regulation of cell growth. The working hypothesis for this aim is that low doses of sphingolipids suppress unlimited cell growth and tumor formation by inhibition of the transcription of beta- catenin targets. The approach will be to determine changes in gene transcription and expression/localization of proteins that are both targets of beta-catenin and sphingolipids in vitro and in vivo, and determine the association of sphingolipid-mediated beta-catenin regulation and transcriptional activity with changes in aberrant cell behavior, and therefore the importance of betat-catenin regulation in tumor suppression by sphingolipids. The proposed research is significant because the reversal of aberrant li-catenin localization and its transcriptional activity are thought to be critical steps in colon cancer prevention. The outcomes of the proposed studies are expected to provide the basis for a colon cancer prevention strategy based on dietary or orally administered sphingolipids.